CCSS.Math.Content.HSG-SRT.A.1
The standard
Verify experimentally the properties of dilations given by a center and a scale factor:
Common Core State Standards for Mathematics
What this standard means
Students need to test what happens when a figure is enlarged or reduced from a fixed point. They should measure distances from the center, compare side lengths, and notice which lines stay in place, which lines move to parallel lines, and how lengths change by the scale factor.
Mastery looks like using a ruler, graph paper, or geometry software to justify patterns with evidence, not just saying “it looks bigger.” Students often mix up the center and the image point, forget that distances from the center scale too, or think all lines move to parallel lines.
Ways to teach it
- Have students dilate a triangle on graph paper from a marked center using scale factors 2 and 1/2, then measure matching sides.
- Prompt students to explain what happens to a line through the center compared with a line not through the center.
- Give an exit ticket with one dilated segment and ask students to find the scale factor and center by tracing rays.
- Use a phone map zoom example, asking which distances change on the screen and which point acts like the center.
Plan a lesson for CCSS.Math.Content.HSG-SRT.A.1
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Related standards
- CCSS.Math.Content.HSG-SRT.A.1b
The dilation of a line segment is longer or shorter in the ratio given by the scale factor.
- CCSS.Math.Content.7.G.A.1
Solve problems involving scale drawings of geometric figures, including computing actual lengths and areas from a scale drawing and reproducing a scale drawing ...
- CCSS.Math.Content.HSG-SRT.A.1a
A dilation takes a line not passing through the center of the dilation to a parallel line, and leaves a line passing through the center unchanged.
- CCSS.Math.Content.8.G.A.1
Verify experimentally the properties of rotations, reflections, and translations: